A patent document 1 (JP 2002-21515 A) discloses a valve control apparatus having an intake cam shaft, an exhaust cam shaft that rotates by receiving a crank torque from a crank shaft of an internal combustion engine, and an intermediate rotor, with which the exhaust cam shaft is provided, rotating relative to the exhaust cam shaft. The rotational phase of the intake cam shaft relative to the crank shaft and the exhaust cam shaft is adjusted when the exhaust cam shaft is rotated in association with the intake cam shaft through the intermediated rotor.
A patent document 2 (JP 4161356 B) discloses a valve control apparatus having an intake cam shaft, a housing rotor that rotates in association with a crank shaft of an internal combustion engine, and an vane rotor, with which the housing rotor is provided, rotating relative to the housing rotor. The rotational phase of the intake cam shaft relative to the crank shaft is adjusted when the housing rotor is rotate in association with the intake cam shaft through the vane rotor. In the valve control apparatus of the patent document 2, the rotational phase of the intake cam shaft relative to the crank shaft is locked at an intermediate phase between the most-retarded phase and the most-advanced phase when the engine is started. According to such an intermediate phase locking mechanism, the startability of the engine can be secured, especially during a cold start under low-temperature environment.
In the valve control apparatus of the patent document 1, the locking mechanism in the valve control apparatus of the patent document 2 can be used. That is, the rotational phase of the intermediate rotor relative to the crank shaft and the exhaust cam shaft can be locked at an intermediate phase by the locking mechanism. However, in a case where the engine is started in a failure state in which the intermediate phase locking is released, such as a state after the engine is stopped in a moment at a phase other than the intermediate phase (i.e., engine stall), the intermediate rotor needs to be rotated to the intermediate phase by the cam torque transmitted to the intermediate rotor from the intake cam shaft. In this case, the intermediate rotor receives the cam torque acting alternately in an advance direction and in a retard direction according to a rotational angle of the crank shaft. Therefore, it may be difficult to keep the intermediated rotor at the intermediate phase, and thus to secure the intermediate phase locking during an engine start, resulting in deteriorating startability of the engine.